1. Field of the Invention
This invention relates to a process for polymerizing olefins onto a chromium-modified filler in the presence of an organoaluminum compound, and to the novel polyolefin/filler compositions obtained thereby.
2. Description of the Prior Art
The utility of organic polymers has been broadened in recent years to the degree that rigid polymers such as the nylons, ABS (acylonitrile/butadiene/styrene) and polyacetal resins have begun to replace the more conventional metal, wood and ceramic materials. The lower cost of polyolefins such as polyethylene could make them candidates for a wider range of applications if certain properties such as heat distortion temperature, stiffness and hardness could be improved.
Much research has been directed toward methods of improving those properties mostly by filling polyolefins with finely divided solids or fibrous fillers. One method of providing filled polyolefins is by melt mixing the polyolefin with the filler. This procedure, however, requires that the polyolefin be of relatively low molecular weight, that is, have an inherent viscosity of less than about 1. While the resulting products generally have increased stiffness, they suffer from the disadvantage of lower elongation and increased brittleness. Serious problems of compounding these polymers are also encountered including the large power requirements for mixing machinery, degradation of polymers by heating, nonuniformity of filler dispersion, and poor adhesion of polymer to filler, even when "coupling compounds" are employed.
Various attempts have been made to decrease these problems by preparing filled polyolefins without compounding the polyolefin and the filler; the most widely used method being polymerization of the olefin in the presence of selected fillers. One suggested method of effecting olefin polymerization on the filler is with coordination catalysts. These well-known catalysts are combinations of halides or esters of titanium or another transition metal including chromium and a reducing compound such as an organoaluminum compound or, more broadly, an organometallic compound of a metal of Group I, II or III of the Periodic Table. This method in general, however, has not provided toughness in highly filled polyolefin compositions.
Specific prior art pertinent to the present invention includes the following:
1. R. J. McManimie, U.S. Pat. No. 3,578,629, prepares filled polyolefins by low pressure polymerization of an .alpha.-monoolefin in the presence of a coordination catalyst and an inorganic filler previously reacted with an organosilane "coupler" compound. A wide range of inorganic fillers is mentioned including kaolinite, attapulgite and bentonite.
2. D. F. Herman et al., U.S. Pat. No. 3,297,466, disclose a continuous process for treating material such as particles of cellulose, silicates, asbestos and other particulate materials with a multicomponent catalyst system, following which the treated material can be mixed with a polymerizable monomer and polymerization effected. The resulting product is 90 to 95% polymer uniformly deposited over the particulate material. The components of the catalyst system are chosen from an organometallic compound such as an alkyl or aryl derivative of lithium, sodium, potassium, magnesium, calcium, zinc, cadmium, boron or aluminum, and a transition metal compound such as a halide or ester of titanium, zirconium, vanadium or chromium.
3. G. G. McClaflin et al. U.S. Pat. No. 3,466,242, disclose the coating of various finely divided solids such as silica flour, fullers earth and sand with 2 to 20% of a polymer obtained by polymerizing a 1-olefin such as ethylene in the presence of the solids which have been treated with a catalytic material, for example, triethylaluminum and titanium tetrachloride. It is stated that compounds of the heavier metals of Group IV-B, V-B or VI-B of the Mendeleev period system of elements can be used with the alkylaluminum compound.